1. Field of the Invention
The invention relates to a two stage catalytic process with stripping, in a single reaction vessel, for purifying a diesel fuel stock. More particularly, the invention relates to a process for selectively removing sulfur and nitrogen compounds from a diesel fuel stock with hydrogen, in a first reaction stage, to produce a sulfur and nitrogen reduced liquid effluent, which is then stripped in a stripping stage and passed into a second reaction stage, in which it reacts with fresh hydrogen in the presence of a catalyst selective for aromatics saturation, to produce a diesel fuel stock reduced in sulfur, nitrogen and aromatics. The reaction and stripping stages are all in the same reaction vessel.
2. Background of the Invention
Many hydrocarbon feeds useful for producing diesel fuel stocks contain undesirable components which must be removed or converted to meet fuel specifications. These components include heteroatoms such as sulfur, nitrogen and oxygen, aromatic unsaturates, halides, and metals. In conventional diesel fuel hydrotreating processes, heteroatom compounds and often aromatic unsaturates are removed from the feed by reacting it with hydrogen in the presence of one or more catalysts. Various conventional process configurations have been developed which use multiple vessels as disclosed, for example, in U.S. Pat. Nos. 5,705,052; 5,720,872; 5,968,346 and 5,985,135.
Specifications for diesel fuels with respect to such undesirable components, particularly sulfur and nitrogen, and also aromatics, are increasingly stringent. Consequently, more upgrading is required to reduce the amount of these components in the diesel stock to the lower levels required by the more stringent specifications. Moreover, as the availability of clean feeds for producing diesel continues to decrease, petroleum refiners will need to rely more heavily on relatively high boiling feeds derived from such materials as coal, tar sands, shale oil, and heavy crudes. Such high boiling feeds typically contain significant amounts of undesirable components. Refiners must therefore not only further reduce the levels of these components in diesel stocks, but must also deal with feeds having higher concentrations of undesirable components. Conventional diesel hydrotreating facilities may therefore have difficulty meeting increasingly stringent diesel specifications, a difficulty that may be further exacerbated by the poorer quality of available feeds.
Factors such as space constraints, which limit or prevent adding additional reaction vessels, strippers, and associated utilities, may impede the construction of new, grass-roots diesel fuel hydroprocessing equipment to increase the purity of the diesel fuel stock, the capacity for diesel stock production, or both. There is therefore a need for cost effectively increasing the purity of a diesel fuel stock produced in an existing diesel fuel processing facility, the facility""s production capacity, or both, without having to add multiple hydrotreating reaction vessels and strippers, for the required hydrotreating reactions and stripping.
In one embodiment, the invention is a process for removing heteroatom compounds and aromatic unsaturates from a hydrocarbon feed boiling in the diesel fuel range comprising:
(a) passing said feed and hydrogen into a first reaction stage, in which they react in the presence of a catalytically effective amount of a first catalyst selective for removal of heteroatom compounds under first catalytic conversion conditions, to produce a first stage effluent comprising (i) a liquid boiling in the diesel fuel range reduced in heteroatom compounds and (ii) a vapor containing vaporous heteroatom species produced by the reaction;
(b) separating said first stage liquid and vapor effluents;
(c) stripping said first stage liquid effluent in a stripping stage to remove dissolved heteroatom species produced by said first stage reaction;
(d) passing said stripped liquid into a second reaction stage, where the stripped liquid reacts with fresh hydrogen or a fresh hydrogen treat gas under second stage catalytic conversion conditions, in the presence of a second catalyst selective for aromatics removal, to remove aromatic compounds and produce second stage liquid and vapor effluents, wherein said vapor contains unreacted hydrogen and said liquid comprises a diesel boiling-range hydrocarbon having a heteroatom and aromatics content lower than that of said feed;
(e) separating the second stage liquid and vapor effluents and recovering said second stage liquid effluent; and
(f) passing said second stage vapor effluent into said first stage, said two reaction stages and said stripping stage being in a common vessel.
In another embodiment, the invention is a method for upgrading an existing diesel hydrotreating facility comprising one or more reaction vessels, strippers and associated equipment and which produces a hydroprocessed diesel fraction, said method comprising:
(a) reacting said hydroprocessed diesel fraction feed and hydrogen in a first reaction stage under first catalytic conversion conditions in the presence of a first catalyst selective for removal of heteroatom compounds, to produce a first stage effluent comprising (i) a liquid effluent containing a diesel fraction reduced in heteroatom compounds and (ii) a vapor effluent containing vaporous heteroatom species produced by the reaction;
(b) separating said first stage liquid and vapor effluents;
(c) stripping said first stage liquid effluent in a stripping stage to remove dissolved heteroatom species produced by said first stage reaction;
(d) passing said stripped liquid into a second reaction stage, in which it reacts with fresh hydrogen or a fresh hydrogen treat gas under second stage catalytic conversion conditions, in the presence of a second catalyst selective for aromatics removal, to remove aromatic compounds and produce second stage liquid and vapor effluents, wherein said vapor effluent contains unreacted hydrogen and said liquid effluent contains a diesel boiling-range hydrocarbon fraction having a heteroatom and aromatics content lower than that of said hydroprocessed diesel fraction fed into the first reaction stage;
(e) separating said second stage liquid and vapor effluents and recovering said liquid effluent; and
(f) conducting said second stage vapor effluent to said first stage, said two reaction stages and said stripping stage being in a common vessel.